RFR: 8299072: java_lang_ref_Reference::clear_referent should be GC agnostic

[Erik Österlund]

The current java_lang_ref_Reference::clear_referent implementation performs a raw reference clear. That doesn't work well with upcoming GC algorithms. It should be made GC agnostic by going through the normal access API.

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Commit messages:
 - 8299072: java_lang_ref_Reference::clear_referent should be GC agnostic

Changes: https://git.openjdk.org/jdk/pull/11736/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11736&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299072
  Stats: 8 lines in 5 files changed: 5 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/11736.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11736/head:pull/11736

PR: https://git.openjdk.org/jdk/pull/11736